In the manufacture of paper from wood, the wood is first reduced to an intermediate stage in which the wood fibers are separated from their natural environment and transformed into a viscous liquid suspension known as a pulp. There are several classes of techniques which are known, and in general commercial use, for the production of pulp from various types of wood. The simpliest in concept of these techniques is the so-called refiner mechanical pulping (RMP) method, in which the input wood is simply ground or abraded in water through a mechanical milling operation until the fibers are of a defined desired state of freeness from each other. Other pulping methodologies include thermo-mechanical pulping (TMP), chemical treatment with thermo-mechanical pulping (CTMP), chemi-mechanical pulping (CMP) and the so-called kraft or sulfate process for pulping wood. In all of these processes for creating pulps from wood, the concept is to separate the wood fibers to a desired level of freeness from the complex matrix in which they are embedded in the native wood.
Of the constituents of wood as it exists in its native state, the cellulose polymers are the predominate molecule which is desired for retention in the pulp for paper production. The second most abundant polymer to cellulose in the native wood, which is the least desirable component in the pulp, is known as lignin. Lignin is a complex macromolecule of aromatic units with several different types of interunit linkages. In the native wood, lignin physically protects the cellulose polysaccharides in complexes known as lignocellulosics, and those lignocellulosics must be disrupted for there to be marked enzyme accessibility to the polysaccharides, or to separate lignin from the matrix of the wood fibers.
It has been suggested that biological systems can be utilized to assist in the pulping of wood. A desirable biological system would be one which is intended to liberate cellulose fibers from the lignin matrix by taking advantage of the natural abilities of a biological organism. Research in this area has focused on a type of fungi referred to as white-rot wood decay fungi. These fungi are referred to as white-rot, since the characteristic appearance of wood infected by these fungi is a pale color, which color is the result of the depletion of lignin in the wood, the lignin having been degraded or modified by the fungi. Since the fungi appear to preferentially degrade or modify lignin, they make a logical choice for fungi to be utilized in biological treatments to pulp wood, referred to as biopulping.
Several reports have been made of attempts to create biopulping systems using white-rot fungi on a variety of wood fibers. Previous research has concentrated on a single, or relatively few, species of fungi. The most commonly utilized fungi in such prior systems is the white-rot fungi Phanerochaete chrysosporium, also referred to as Sporotrichum pulverulentum. Other fungi which have been previously used in such procedures include fungi of the genera Polyporus and Phlebia. The prior art is generally cognizant of the fact that attempts have been made to use biological organisms, such as white-rot fungi, as part of a process of treating wood, in combination with a step of either mechanical or thermal mechanical pulping of cellulose fiber.
An example of a teaching of a method of producing cellulose pulp is shown in U.S. Pat. No. 3,962,033 directed to the biopulping of cellulose utilizing white-rot fungi. The fungi utilized included both of naturally occurring wild-type strain cultures and also mutant strains produced which were intended to be cellulase-less, so as to reduce the amount of cellulose degraded by the organisms. Various types of wood were degraded with the fungi, and then were used as input materials for a thermo-chemical or mechanical pulping procedure. This patent disclosure discusses various techniques for making a cellulose pulp by depleting lignin while reducing the cellulose-decomposing action of the enzymes produced by these organisms in order to preserve the cellulose yield. Groups working with the inventors of said patent also had several other publications regarding use of such fungi for biomechanical pulping, e.g. Anders and Erikkson, Svensk Papperstidning, 18:641-2 (1975), Erikkson and Vallander, Svensk Papperstidning, 6:85:33-38 (1982). Unfortunately, the process never gained commercial acceptance, and has not been widely utilized. One of the difficulties is that most of the prior techniques for utilizing biological techniques for the pulping of paper have resulted in paper which has had only marginal strength increase or is weaker than papers made by more conventional processes.
In fact, while a certain amount is known about the interaction of lignin and cellulose in wood fibers, because of the extreme complexity of the relationships, and the variation in the enzymes produced by varieties of the white-rot fungi, it is not readily possible to predict from the action of a given fungi on wood whether or not the paper made from wood partially digested with such fungi will have desirable qualities or not. The selection of white-rot fungi for biopulping applications on the basis of selective lignin degradation may seem a rational one, but it has proven to be a poor predictor of the quality of the resultant paper. The exact relationship between the degradation of lignin, and the resulting desirable qualities of paper produced at the end of the pulping process, are not at all clear. Accordingly, given present standards of technology and the present understanding of the complex interaction of lignin and cellulose, it is only possible to determine empirically the quality of paper produced through a given biological pulping process and the amount of any energy savings achieved through such a process.